Temperature plays a major role in the rate at which therespiratory reactions occur. For example, when airtemperatures rise from 20°C (68°F) to 30°C (86°F), therespiration rates of plants double and sometimes even triple. Respiration is a catabolic process that takes place in thecytoplasm and mitochondria of cells. The energy is released,with the aid of enzymes, from simple sugar and organic acidmolecules. In aerobic respiration, stored energy release requiresoxygen; CO2 and water are by-products of the process.Anaerobic respiration and fermentation do not requireoxygen gas, and much less energy is released. The remainingenergy is in the ethyl alcohol, lactic acid, or other suchsubstances produced. Some released energy is stored in ATPmolecules. Temperature, available water, and environmentaloxygen affect respiration rates.

Glycolysis, which occurs in the cytoplasm, requires nomolecular oxygen; two phosphates are added to a 6-carbonsugar molecule, and the prepared molecule is split into two3-carbon sugars (GA3P). Some hydrogen, energy, andwater are removed from the GA3P, producing pyruvicacid. There is a net gain of two ATP molecules. Hydrogenions and electrons released during glycolysis are picked upby NAD, which becomes NADH.In aerobic respiration, which occurs in themitochondria, pyruvic acid loses some CO2, isrestructured, and becomes acetyl CoA. Energy, CO2, andhydrogen are removed from the acetyl CoA in the citricacid cycle, which involves enzyme-catalyzed reactions of aseries of organic acids.

Aerobic respiration

Anaerobic respiration

2

2

NADH passes the hydrogen gained during glycolysis and thecitric acid cycle along an electron transport system; smallincrements of energy are released and partially stored in ATPmolecules, and the hydrogen is combined with oxygen gas,forming water in the final step of aerobic respiration (electrontransport chain). Several of the electron carriers in the transportsystem are cytochromes. They are very specific and, as electronsflow along the system, they can transfer their electrons only toother specific acceptors. When the electrons reach the end of thesystem, they are picked up by oxygen and combine with hydrogenions, forming water. Hydrogen removed from glucose during glycolysis is typicallycombined with an inorganic ion in anaerobic respiration. Thehydrogen is combined with the pyruvic acid or one of itsderivatives in fermentation. Both processes occur in the absenceof oxygen gas, with only about 7% of the total energy in theglucose molecule being released, for a net gain of two ATPmolecules.

Two molecules of NADH and two ATP molecules are gainedduring glycolysis when two 3-carbon pyruvic acid molecules areproduced from a single glucose molecule. Another molecule ofNADH is produced when the pyruvic acid molecule is restructuredand becomes acetyl CoA prior to entry into the citric acid cycle. In the citric acid cycle, acetyl CoA combines with 4-carbonoxaloacetic acid, producing first a 6-carbon compound, next a 5-carbon compound, and then several 4-carbon compounds. The last4-carbon compound is oxaloacetic acid. Two CO2 molecules arealso released during this process. Some hydrogen removed during the citric acid cycle is picked upby FAD and NAD; one molecule of ATP, three molecules of NADH,and one molecule of FADH2 are produced during one completecycle. Energy associated with electrons and/or with hydrogenpicked up by NAD and FAD is gradually released as the electronsare passed along the electron transport system; some of this energyis transferred to ATP molecules during oxidative phosphorylation.

Energy used in ATP synthesis during oxidative phosphorylation isbelieved to be derived from a gradient of protons formed across theinner membrane of a mitochondrion, while electrons are moving inthe electron transport system by chemiosmosis.Altogether, 38 ATP molecules are produced during the completeaerobic respiration of one glucose molecule; two are used to primethe process, so there is a net gain of 36 ATP molecules. In addition to photosynthesis and respiration, other metabolicpathways are required for growth, development, reproduction, andsurvival. Essential products of additional pathways includenucleotides, proteins, chlorophylls, and fatty acids. Secondarymetabolites include alkaloids, phenolics, and terpenoids. Conversion of sugar produced by photosynthesis to fats, proteins,complex carbohydrates, and other substances is termed assimilation.Digestion takes place within plant cells with the aid of enzymes.During digestion, large insoluble molecules are broken down byhydrolysis to smaller soluble forms that can be transported to otherparts of the plant.

Growth is defined as an “irreversible increase in volume due to thedivision and enlargement of cells.” As cells mature, they differentiate intoforms adapted to specific functions. Development is a change in form as a result of growth anddifferentiation combined. Cells themselves assume different shapes andforms, which adapt them to the problem involved in their total volumeincreasing at a greater rate than their total surface area.Many growth phenomena are influenced by hormones, which are

produced in minute amounts in one part of an organism and usuallytransported to another part, where they have specific effects on growth,flowering, and other plant activities. Vitamins are organic molecules thatfunction in activating enzymes; they are sometimes difficult to distinguishfrom hormones. The major known types of plant hormones are auxins,gibberellins, cytokinins, abscisic acid, and ethylene. Darwin and his son noted in 1881 that coleoptiles bend toward a lightsource. Frits Went, in 1926, followed up on the Darwinian observationsand demonstrated that something in coleoptile tips moved out into agarwhen decapitated tips were placed on it. He called the substance auxin andshowed that it could cause coleoptiles to bend.

Went’s experiment with oat coleoptiles. A. A germinated oat “seed” with an intactcoleoptile. B. The tip of a coleoptile was cut off, placed on a small agar block, and left for anhour or two. C. When this agar block was placed squarely on a decapitated coleoptile,growth was vertical. D. When the agar block was placed off center so that only half of thedecapitated coleoptile was in contact with it, the tip bent away from it. The leaf within thecoleoptile was pulled up slightly to support the agar block. This experiment demonstratedthat something affecting growth diffused from the coleoptile tip into the agar and from theagar to the part of the decapitated coleoptile touching the block.

How a bioassay for auxin is made. A. Coleoptile tips are cut and placed on a measuredportion of agar. B. for a set period of time. C. The coleoptile tips are then removed, and theagar is cut into blocks of specific size. An agar block is placed off center on a decapitatedcoleoptile held by a clamp; the leaf within the coleoptile is pulled up slightly to support theagar block. After another set period, the angle of curvature is measured. D. The angle ofcurvature is compared to that produced when a similar agar block containing a knownamount of auxin is placed off center on another coleoptile for the same period of time.

Three groups of plant hormones that promote the growth of plants havebeen found; others are known only for their inhibitory effects. Auxinsstimulate the enlargement of cells and are involved in many other growthphenomena. Synthetic auxinlike compounds (2,4,5-T & 2,4-D) have beenused as weed killers and defoliants. IAA or auxin (indole acetic acid)stimulate the formation of roots on almost any plant organ. Monocotplants tends to be the least sensitive to auxins.Acetyl coenzyme A, which is vital to the process of respiration, functions asa precursor in the synthesis of GA. ABA is synthesized in plastids, apparentlyfrom carotenoid pigments. Gibberellins promote stem growth without corresponding root growth andflowering; cytokinins promote cell division and can be used to stimulate thegrowth of axillary buds. Abscisic acid causes buds to become dormant andapparently helps leaves respond to excessive loss of water. Ethylene gas hastensripening of fruits and is used commercially to ripen green fruits. Senescence is the breakdown of cell parts that leads to the death of the cell. Plant movements, such as nutations (spiraling growth) and nodding (thebent hypocotyl – growing tip seedling through the soil), twining, contractile,and nastic movements (turgor changes or pressure in special cells - nasticinclude sleep and contact movements), are growth movements that resultprimarily from internal stimuli.

Typical twining of a tendril produced by amanroot plant. Note that the direction ofcoiling reverses near the midpoint.

The leaflets of the leaf toward the bottomof this picture have folded upward inresponse to being bump. The other leavesof this sensitive plant have remained fullyexpanded.

Effect of auxin applied to the base of a Gardenia cutting 4 weeks after application.Left. A cutting treated with auxin. Right. An untreated cutting.

Apical dominance is thesuppression of the growth ofthe lateral buds (also calledaxillary buds), each of whichcan form a branch with its ownterminal bud. Apicaldominance is believed to bebrought about by an auxinlikeinhibitor in a terminal bud. Itis strong in trees with conicalshapes and little branchingtoward the top.Apical dominance can be offsetwith an application ofcytokinins to axillary buds.A Jeffrey pine tree. The trunkof this tree, which wouldnormally be single, is forkedbecause of the earlier removalof the terminal bud, allowing alateral bud to grow.

Effect of gibberellinson flowering. Cabbageplants on the rightwere grown in a warmgreenhouse but did notgrow tall and floweruntil treated withgibberellins. The shortcabbage plants on theleft ere not treatedwith gibberellins.

Effect of ethylene on holly twigs. Two similar twigs were placed under glass jarsfor a week; at the same time, a ripe apple was placed under the jar on the right.Ethylene produced by the apple caused abscission of the holly leaves.

Water conservation movement in a grass leaf when insufficient water tomaintain normal turgor is available. A. The leaf when adequate water isavailable. B. The leaf after it has rolled up. C. Enlargement of a crosssection of a rolled leaf showing the location of the large, thin-walledbulliform cells, which partially collapse under dry conditions and thusbring about the rolling of the leaf blade.

Tropisms are permanent, directed growth movements thatresult from external stimuli, such as light (phototropism), gravity(gravitropism), contact (thigmotropism), and chemicals such assalt (chemotropism or halotropism). Turgor movements result from changes in internal waterpressures; they may be very rapid or take up to 45 minutes tobecome visible. Turgor movements include contact movementsand sleep movements in which leaves or flowers fold daily in whatis known as a circadian rhythm. Garner and Allard’s were responsible for coining the termphotoperiodism. Photoperiodism is a response of plants to the duration of dayor night. Short-day plants will not flower unless the day length isshorter than a critical day length, and long-day plants will notflower unless the day length is longer. Intermediate-day plantshave two critical photoperiods; the flowering of day-neutralplants is independent of day length.

A bush monkey flower. The white, two-lobed structure in the centeris the stigma. A. The stigma as it appears before pollination. B. Thestigma 2 seconds after being touched by a pollinator; the lobes haverapidly folded together.

A prayer plant (Maranta). A. The plant at noon. B. The same plant at 10 P.M., after “sleep” movements of its leaves have occurred.

Photoperiodism. The poinsettia with red bracts received lessthan 8.5 hours of light per day. The green poinsettias received more than 10 hours of light per day and did not flower.

A cyclamen plant thatreceived light from onedirection for severalweeks. Note how all visibleplant parts are orientedon the side that receivedlight. This is an exampleof positive phototropism.

It was believed that stemtips bent toward lightbecause auxin wasdestroyed or inactivated onthe exposed side, leavingmore growth-promotinghormone on the side awayfrom the light, causing thecells there to elongate moreand produce a bend.

An experiment illustrating the effect of subjecting one leaf of a short-dayplant (cocklebur) to short days while the rest of the plant is exposed to longdays. A. The short-day plant exposed to long days. No flowers wereproduced. B. The same plant exposed to the same long days while one leafwas covered with black paper 16 hours a day for a few weeks (mimickingshort days). The plant flowered, presumably because some substance thatinitiates flowering was produced in the shaded leaf and then diffused orwas carried to the stem tip where flower buds are produced.

Phytochromes are light-sensitive pigments that occur in allhigher plants and play a role in many different plant responses(flowering). Phytochromes occur in two forms, each of which can beconverted to the other by the absorption of light. Day light generallyresults in more Pr being converted to Pfr (the active form) than viceversa. Pr absorbs red light and Pfr absorbs far-red light. Pfr willconvert back to Pr over a period of several hours in the dark; Pr isstable indefinitely in the dark.

Growth responses to the stimulus of gravity are called gravitropisms. Theprimary roots of plants tend to be positively gravitropic, while shoots formingthe main axis of plants are typically negatively gravitropic. Dormancy is a period of growth inactivity in seeds, buds, bulbs, and otherplant organs even when appropriate environmental conditions are met.Quiescence is a state in which a seed is unable to germinate unless appropriateenvironmental conditions exist.

This Coleus plant was placed on its side the day before the photograph was taken.The stems bent upward within 24 hours of the pot being tipped over. This is anexample of negative gravitropism.